The present invention relates to the encoding and decoding of video information and, more particularly, to a method and system for secure transmission of television signals for subscription television or similar video services in which only authorized viewers are permitted to view a video program.
With the increased interest and activity in the field of subscription or pay video transmission of all types (e.g., broadcast and cable subscription television, long distance satellite transmission, television transmission of textual information, etc.), there has arisen a need for more secure transmission of high quality video information. To this end, numerous video encoding or scrambling techniques have been developed and some are now in use, particularly in broadcast systems where there is no control over who receives the signals and thus the signals must be encoded to prevent unauthorized use of the received signals.
One known type of broadcast subscription television system transmits a video signal that is modulated by a sine wave signal such that the blanking and synchronizing levels cannot be recognized by a normal television receiver. The display, without decoding, is unintelligible or at least very annoying to a viewer. However, by rather simple means that are readily available to an average consumer, the system can be easily defeated by non-subscribers. As this becomes more widely known to the public, the number of unauthorized viewers grows and the incentive to pay for the services diminishes. This, in turn, detracts from the desirability on the part of video program producers to permit the use of their programming, particularly if they are paid as a function of authorized viewers or revenues collected.
Other approaches to television signal scrambling have proven to be more secure and may, in fact, make it practically impossible to unscramble the video signals without highly sophisticated and extremely expensive equipment. One such approach is to invert lines or fields of video information on some basis that can be reproduced at the subscriber location to permit viewing of a normal picture. Thus, for example, one known system inverts alternate parts of the video information and a decoder at the subscriber location can reinvert those inverted portions to reconstitute the original video. Another known system inverts fields of video information on a random basis and sends a code with the scrambled video to instruct the decoder as to how the received video has been inverted. An example of this latter type of system is disclosed in U.S. Pat. No. 4,024,576.
Security tends to be adequate in approaches to video scrambling in which the video information is randomly inverted and a secure code is transmitted with the video so that the decoder can properly reinvert, but difficulties arise with respect to picture quality. For example, inversion and reinversion of video signals may result in a reconstituted video signal that varies in d.c. level from line-to-line or field-to-field. Because of this variation, a flicker or other annoying effect appears in the television display making it unpleasant for viewing.
Various measures have been employed to eliminate or at least reduce this problem with varying degrees of success. Clamping the video signals to the same d.c. level has been somewhat successful, but some annoying effects may still remain. Also, the additional circuitry required to eliminate or reduce the annoying effects of these types of scrambling add cost and complexity to the decoders. Less expensive and perhaps more effective approaches, such as less frequent inversion, have been suggested, but they seem to have a tendency to reduce the security of transmission and only serve to reduce the annoyance, not eliminate it.
It is accordingly a general object of the present invention to provide a novel method and system for scrambling or encoding television signals wherein the difficulties of the prior art systems mentioned above are obviated.
It is a further object of the present invention to provide a novel television video encoding and decoding method and system in which there is an extremely high amount of security, wherein the encoding is sufficient to render an encoded television signal unintelligible or at least extremely annoying to watch, and wherein the decoded video signal is devoid of annoying abnormalities introduced during encoding, yet no special efforts must be made to eliminate encoding abnormalities.
It is another object of the present invention to provide a novel television signal scrambling system particularly suitable for broadcast pay television or other television systems meant only for authorized users.
One system that is directed to achieving these general objectives is disclosed in copending application Ser. No. 124,656, filed on Feb. 25, 1980 by Robert S. Block et al. In the system disclosed therein, the video signal is encoded by delaying parts of the signal relative to other parts in a determinable manner to thereby rearrange the sequence of transmission of the parts in a fashion that can be reproduced at a remote location. Generally, this rearrangement is accomplished by storing successive parts of the video signal and retrieving the stored parts in an order or sequence other than their normal sequence.
The rearrangement of the sequence of video signal parts can be accomplished on a line basis or a field basis. In the specific embodiments of the invention disclosed in the previously mentioned patent application, each part of the video signal that is to be delayed relative to another part comprises one or more digital samples of the signal. For example, each horizontal line of the signal can be divided into 256 segments of digital information that are stored and rearranged relative to one another to thereby scramble the signal. The rearrangement can be accomplished by the manner in which the segments are stored in a storage device such as a shift register, or the manner in which it is retrieved therefrom.
The system disclosed in the noted application has been determined to be successful in achieving the foregoing objects. It is an objective of the present invention to provide a method and system for secure transmission of video signals that is an improved version of that system, particularly from the standpoints of cost and security. More specifically, when a video signal is digitized prior to rearrangement of its various parts, the hardware necessary to carry out the analog-to-digital conversion of the signal is relatively expensive, in comparison with the other components of the signal encoding or decoding system. Consequently it is desirable to be able to rearrange the parts of the signal while they are in an analog format, to thereby forego with the expenses associated with the conversion of the signal into digital samples prior to scrambling, and subsequent reconversion of the scrambled signal into analog form for transmission.
Furthermore, a digital implementation of the scrambling technique imposes a practical limitation on the portions of the signal that can be rearranged. When the video information contained in each horizontal line of the signal is digitized and subsequently reconverted to analog form, the information can be recaptured relatively error free. However, because of the nature of the synchronization pulses and associated timing information in each line of the video signal, any reliable digitization and subsequent reconversion of this portion of the signal requires relatively complex and expensive equipment when utilizing presently-known technology. Consequently, in a practical system the video and synchronization portions of the signal are separated, and only the video information portion of each line of the signal is digitized and scrambled. The synchronization information is not encrypted and is recombined with the scrambled video portion of the signal before transmission.
It is a more specific object of the present invention to provide a novel method and system for encoding and decoding a television signal that performs the encoding and decoding operations while the signal is in an analog format, to thereby avoid the costs associated with converting a signal into digital form and subsequently reconverting it into an analog signal.
It is yet another object of the present invention to provide a novel method and system of this type in which various portions of a video signal are rearranged relative to each other so that both the synchronization pulses and the video information are not suitable for use by a conventional television receiver while in their scrambled form.
It is still a further object of the present invention to provide a novel system for controlling the encoding and decoding of a television signal with a pseudo-random control signal that is not transmitted with the television signal.
It is still another object of the present invention to provide a highly secure system for controlling the encoding and decoding of a television signal with a code that is generated independently at both the transmitting and receiving ends of a television system in accordance with a control word that is transmitted in an encrypted form with the television signal.